The present invention relates in general to cardiac stimulation devices, such as pacemakers, defibrillators, cardioverters, implantable cardioverter-defibrillators (xe2x80x9cICDsxe2x80x9d), and similar cardiac stimulation devices that are capable of monitoring and detecting electrical activities and events within the heart. In particular, this invention pertains to a system and method for automating the detection of capture on a ventricular channel of an implantable dual-chamber stimulation device, using far-field evoked response sensed on an atrial channel.
Implantable medical devices, such as pacemakers, defibrillators, cardioverters, and implantable cardioverter-defibrillators (xe2x80x9cICDsxe2x80x9d), collectively referred to herein as implantable cardiac stimulating devices, are designed to monitor and stimulate the heart of a patient who suffers from a cardiac arrhythmia. Using leads connected to a patient""s heart, these devices typically stimulate the cardiac muscles by delivering electrical pulses in response to measured cardiac events that are indicative of a cardiac arrhythmia. Properly administered therapeutic electrical pulses often successfully reestablish or maintain the heart""s regular rhythm.
Implantable cardiac stimulating devices can treat a wide range of cardiac arrhythmias by using a series of adjustable parameters to alter the energy, shape, location, and frequency of the therapeutic pulses. The adjustable parameters are usually defined in a computer program stored in a memory of the implantable device. The program, which is responsible for the operation of the implantable device, can be defined or altered telemetrically by a medical practitioner using an external implantable device programmer.
Programmable pacemakers are generally of two types: (1) single-chamber pacemakers, and (2) dual-chamber pacemakers. In a single-chamber pacemaker, the pacemaker provides stimulation pulses to, and senses cardiac activity within, a single-chamber of the heart, either the right ventricle or the right atrium. In a dual-chamber pacemaker, the pacemaker provides stimulation pulses to, and senses cardiac activity within, two chambers of the heart, namely both the right atrium and the right ventricle. The left atrium and left ventricle can also be sensed and paced, provided that suitable electrical contacts are effected therewith.
Pacemakers also have a great number of adjustable parameters that must be tailored to a particular patient""s therapeutic needs. One adjustable parameter of particular importance in pacemakers is the pacemaker""s stimulation energy. For the pacemaker to perform its intended function, it is critically important that the delivered electrical stimuli be of sufficient energy to depolarize the cardiac tissue, a condition known as xe2x80x9ccapturexe2x80x9d.
When a pacemaker stimulation pulse stimulates either the atrium or the ventricle during an appropriate portion of a cardiac cycle, it is desirable to have the heart properly respond to the stimulus provided. Every patient has a xe2x80x9ccapture thresholdxe2x80x9d which is generally defined as the minimum amount of stimulation energy necessary to effect capture. Capture should preferably be achieved at the lowest possible energy setting yet provide enough of a safety margin so that, if a patient""s threshold increase, the output of an implantable pacemaker, i.e. the stimulation energy, will still be sufficient to maintain capture. Dual-chamber pacemakers may have differing atrial and ventricular stimulation energy that correspond to atrial and ventricular capture thresholds, respectively.
Earlier pacemakers had a predetermined and unchangeable stimulation energy, which proved to be problematic because the capture threshold is not a static value and may be affected by a variety of physiological and other factors. For example, certain cardiac medications may temporarily raise or lower the threshold from its normal value. In another example, fibrous tissue that forms around pacemaker lead heads within several months after implantation may raise the capture threshold.
As a result, some patients may eventually suffer from loss of capture, as their pacemakers were unable to adjust the pre-set stimulation energy to match the changed capture thresholds. One attempted solution was to set the level of stimulation pulses fairly high so as to avoid loss of capture due to a change in the capture threshold. However, this approach may result in some discomfort to patients who were forced to endure unnecessarily high levels of cardiac stimulation. Furthermore, such stimulation pulses would consume extra battery resources, thus shortening the useful life of the pacemaker.
When programmable pacemakers were developed, the stimulation energy was implemented as an adjustable parameter that could be set or changed by a medical practitioner. Typically, such adjustments were effected by the medical practitioner using an external programmer capable of communication with an implanted pacemaker via a magnet applied to a patient""s chest or via telemetry. The particular setting for the pacemaker""s stimulation energy was usually derived from the results of extensive physiological tests performed by the medical practitioner to determine the patient""s capture threshold, from the patient""s medical history, and from a listing of the patient""s medications. While the adjustable pacing energy feature proved to be superior to the previously known fixed energy, some significant problems remained unsolved. In particular, when a patient""s capture threshold changed, the patient was forced to visit the medical practitioner to adjust the pacing energy accordingly.
To address this pressing problem, pacemaker manufacturers have developed advanced pacemakers that are capable of determining a patient""s capture threshold and automatically adjusting the stimulation pulses to a level just above that which is needed to maintain capture. This approach, called xe2x80x9cautocapturexe2x80x9d, improves the patient""s comfort, reduces the necessity of unscheduled visits to the medical practitioner, and greatly increases the pacemakers battery life by conserving the energy used to generate stimulation pulses.
However, many of these advanced pacemakers require additional circuitry and/or special sensors that must be dedicated to capture verification. This requirement increases the complexity of the pacemaker system and reduces the precious space available within a pacemaker""s casing, and also increases the pacemaker""s cost. As a result, pacemaker manufacturers have attempted to develop automatic capture verification techniques that may be implemented in a typical programmable pacemaker without requiring additional circuitry or special dedicated sensors.
A common technique used to determine whether capture has been effected is monitoring the patient""s cardiac activity and searching for the presence of an xe2x80x9cevoked responsexe2x80x9d following a stimulation pulse. The evoked response is the response of the heart to the application of a stimulation pulse. The patient""s heart activity is typically monitored by the pacemaker by keeping track of the stimulation pulses delivered to the heart and examining, through the leads connected to the heart, electrical signals that are manifest concurrent with depolarization or contraction of muscle tissue (myocardial tissue) of the heart. The contraction of atrial muscle tissue is evidenced by generation of a P-wave, while the contraction of ventricular muscle tissue is evidenced by generation of an R-wave (sometimes referred to as the xe2x80x9cQRSxe2x80x9d complex). When capture occurs, the evoked response is an intracardiac P-wave or R-wave that indicates contraction of the respective cardiac tissue in response to the applied stimulation pulse. For example, using such an evoked response technique, if a stimulation pulse is applied to the ventricle (hereinafter referred to as an V-pulse), a response sensed by ventricular sensing circuits of the pacemaker immediately following the application of the V-pulse is presumed to be an evoked response that evidences capture of the ventricle.
However, it is for several reasons very difficult to detect a true evoked response. First, because the ventricular evoked response is a relatively small signal, it may be obscured by a high energy V-pulse and therefore difficult to detect and identify. Second, the signal sensed by the pacemaker""s sensing circuitry immediately following the application of a stimulation pulse may be not an evoked response but noise, such as electrical noise caused, for example, by electromagnetic interference, or myocardial noise caused by random myocardial or other muscle contraction.
Another signal that interferes with the detection of an evoked response, and potentially the most difficult for which to compensate because it is usually present in varying degrees, is lead polarization. A lead/tissue interface is that point at which an electrode of the pacemaker lead contacts the cardiac tissue. Lead polarization is commonly caused by electrochemical reactions that occur at the lead/tissue interface due to application of an electrical stimulation pulse, such as a V-pulse, across the interface.
Because the evoked response is sensed through the same lead electrodes through which the stimulation pulses are delivered, the resulting polarization signal, also referred to as an xe2x80x9cafterpotentialxe2x80x9d, formed at the electrode can corrupt the evoked response that is sensed by the sensing circuits. This undesirable situation occurs often because the polarization signal can be three or more orders of magnitude greater than the evoked response. Furthermore, the lead polarization signal is not easily characterized; it is a complex function of the lead materials, lead geometry, tissue impedance, stimulation energy and other variables, many of which are continually changing over time.
In each of the above cases, the result may be a false positive detection of an evoked response. Such an error leads to a false capture indication, which in turn, leads to missed heartbeats, a highly undesirable and potentially life-threatening situation. Another problem results from a failure by the pacemaker to detect an evoked response that has actually occurred. In that case, a loss of capture is indicated when capture is in fact present, also an undesirable situation that will cause the pacemaker to unnecessarily invoke the pacing energy determination function in a chamber of the heart.
Automatic pacing energy determination is only invoked by the pacemaker when loss of ventricular capture is detected. An exemplary automatic ventricular pacing energy determination procedure has been performed as follows. When loss of ventricular capture is detected, the pacemaker increases the V-pulse output level to a relatively high predetermined testing level at which capture is certain to occur, and thereafter decrements the output level until ventricular capture is lost. The ventricular pacing energy is then set to a level slightly above the lowest output level at which ventricular capture was attained. Thus, ventricular capture verification is of utmost importance in proper determination of the ventricular pacing energy.
When a ventricular stimulation pulse is properly captured in the ventricle, a subsequent ventricular contraction results in a far-field evoked response which may be sensed through an atrial lead, as a xe2x80x9cfar-fieldxe2x80x9d signal, also referred to herein as xe2x80x9cfar-field R-wavexe2x80x9d or FFR. The far-field R-wave confirms successful ventricular capture because the ventricular contraction only occurs after a properly captured ventricular stimulation pulse.
However, previously known dual-chamber pacemakers do not sense ventricular activity through the atrial lead for a particular interval of time (i.e., the xe2x80x9crefractoryxe2x80x9d period) subsequent to the delivery of the ventricular stimulation pulse. Furthermore, the polarization signal formed at the ventricular lead electrode may obscure and/or distort the evoked response signal, even if it were sensed.
It would thus be desirable to provide a system and method for automating the detection of capture on a ventricular channel of an implantable dual chamber stimulation device, with increased accuracy. It would also be desirable to provide a system and method for reducing the negative effect of polarization and noise on capture verification. It would further be desirable to enable the pacemaker to perform ventricular capture verification without requiring dedicated circuitry and/or special sensors.
One feature of the present invention is to address the disadvantages and limitations discussed above. In accordance with this invention, a system and method are provided for automating the detection of capture on a ventricular channel of an implantable dual chamber stimulation device, using far-field evoked response that follows a successfully captured ventricular stimulation pulse, and which is sensed on an atrial channel.
The system and method of the present invention compensate for effects of polarization and noise on the far-field R-wave (FFR) and do not require the use of special dedicated circuitry or special sensors to implement the automated procedure.
The present invention provides an implantable medical device (hereinafter xe2x80x9cpacemakerxe2x80x9d) equipped with cardiac data acquisition capabilities. A preferred embodiment of the pacemaker of the present invention includes a control system for controlling the operation of the pacemaker, a set of leads for receiving atrial and ventricular signals and for delivering atrial and ventricular stimulation pulses, a set of sensing circuits comprised of sense amplifiers for sensing and amplifying the atrial and ventricular signals, a sampler, such as an A/D converter, for sampling atrial and/or ventricular signals, and pulse generators for generating the atrial and ventricular stimulation pulses. In addition, the pacemaker includes memory for storing operational parameters for the control system, such as atrial or ventricular signal sampling parameters, and atrial or ventricular signal samples. The pacemaker also includes a telemetry circuit for communicating with an external programmer.
In a preferred embodiment, the pacemaker control system periodically performs a ventricular capture verification test and, when necessary, a ventricular pacing threshold assessment test, which performs an assessment of the stimulation energy in the ventricular chamber of the patient""s heart. The frequency with which these tests are performed are preferably programmable parameters set by the medical practitioner using an external programmer when the patient is examined during an office visit or remotely via a telecommunication link. The appropriate testing frequency parameter will vary from patient to patient and depend on a number of physiologic and other factors. For example, if a patient is on a cardiac medication regimen, the patient""s ventricular capture threshold may fluctuate, thus requiring relatively frequent testing and adjustment of the atrial stimulation energy. Preferably the system and method of the present invention are implemented in a dual-chamber pacemaker.
In one embodiment of the invention, the pacemaker delivers a ventricular stimulation pulse and then samples a resulting far-field R-wave (xe2x80x9cFFRxe2x80x9d) of the evoked response on the atrial channel during a predetermined far-field interval (xe2x80x9cFFIxe2x80x9d) window. The FFI window starts a short time (e.g. ranges between 10 ms and 50 ms) after the ventricular pulse is initiated and continues for a predetermined period (e.g. ranges between 50 ms and 125 ms).
The pacemaker then compares the FFR sample to a predetermined far-field signal recognition template to verify whether the FFR sample morphology corresponds to a far-field R-wave that is expected to follow a successfully captured ventricular stimulation pulse. If the FFR sample is approximately equal to the far-field signal recognition template, then ventricular capture is deemed verified. Otherwise, the pacemaker performs a ventricular stimulation energy determination procedure. The far-field interval window and the far-field signal recognition template may be predefined by the medical practitioner and stored in the pacemaker memory.
According to one embodiment of the present invention, a control system of the pacemaker sets the far-field interval window to approximately 100 msec. The delivery of the stimulation pulse initiates a post ventricular atrial blanking period on the atrial channel, and the far-field interval window is initiated during the post ventricular atrial blanking period.
The system and method of the present invention thus automatically verify ventricular capture and, when necessary, automatically determine a proper ventricular stimulation energy of the patient""s pacemaker, without requiring dedicated or special circuitry and/or sensors. The system and method allow ventricular beat-by-beat autocapture in dual chamber pacemakers where lead characteristics could prevent reliable detection of the ventricular evoked response on the ventricular lead. In addition, the system and method enable the pacemaker to automatically choose the best method for determining ventricular capture without compromising the patient""s safety, and to reduce the pacemaker energy consumption by allowing an autocapture algorithm to be executed.